Recent advances in ceramic matrix composite (CMC) technology is opening up new applications. Traditionally, these materials have been very costly to produce and had exhibited relatively low strength and toughness. Recent advances have reduced manufacturing costs and improved the strength and toughness of these material systems. These improvements along with the ability of CMCs to perform at elevated temperatures makes the use of CMCs viable for use in aircraft engines and other high temperature applications. CMCs offer the potential for lower weight components and the use of higher operating temperatures than can be achieved with traditional metallic components.
CMC and metallic components that make up an airplane may be subjected to extreme thermal conditions, wherein the structure must be capable of withstanding relatively high thermal loads in a variety of conditions. Parts of the engines, in particular, may be subjected to temperatures in excess of 1300° F. Due to its strength-to-weight ratio and its resistance to thermal stresses CMC materials are increasingly used in such parts. The joining of CMC and metallic components presents a problem, however, as CMCs in general have a much lower coefficient of thermal expansion (CTE) than metals. This results in thermal stresses at joints between the CMC and metallic components, which in turn could lead failure of the CMC component.
One component that is of particular concern is the engine exhaust nozzle. Generally, airplane engine exhaust nozzles have a fixed exit area. In the past the exhaust nozzle has been made of metal, but in the continuing effort to shed excess weight and enable higher gas temperatures, engine exhaust nozzles using CMC materials are now being investigated. Implementing a CMC nozzle faces several challenges. Nozzles are generally made in a single piece. As the engine temperature increases, the metallic engine interface expands at a greater rate than the CMC exhaust nozzle, resulting in thermal stresses that can cause failure of the CMC component. Thermal gradients through the wall thickness also induce high stresses in a continuous hoop (or ring) structure (as an exhaust nozzle) limiting the structural capability. Finally, although CMCs are more resistant to cracking than monolithic ceramics, they are still much more prone to damage than metallic structures.